NetBeans: set explicit Java version for project

NetBeans: set explicit Java version for project

The default behaviour of a NetBeans project is to use the
platform-default Java version. If this differs from the version the
project is to run on there could be unexpected bugs such as
accidentally using standard library classes and/or methods that only
exist in the later Java version.

For example, this will prevent bugs creeping in when the platform-default
is Java 1.8 but the project is to be built and run on Java 1.7.